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PRICE 25 CENTS 


CASEHARDENING 

METHODS EMPLOYED IN THE AUTOMOBILE, 
BICYCLE, BALL AND ROLLER BEARING AND 
ALLIED TRADES-CASEHARDENING SHAFT¬ 
ING-NEW CASEHARDENING METHODS — 
CASEHARDENING BY GAS 



MACHINERY’S REFERENCE BOOK NO. 141 

PUBLISHED BY MACHINERY, NEW YORK 








MACHINERY’S REFERENCE BOOKS 

This book is one of a remarkably successful series of 25-cent Reference Books 
listed below. These books were originated by Machinery and comprise a complete 
working library of mechanical literature, each book covering one subject. The price 
of each book is 25 cents (one shilling) delivered anywhere in the world. 


CLASSIFIED LIST OF REFERENCE BOOKS 


GENERAL MACHINE SHOP PRACTICE 


No. 7. Lathe and Planer Tools. 

No. 10. Examples of Machine Shop Practice. 

No. 25. Deep Hole Drilling, 

No. ,32. Screw Thread Cutting. 

No. 48. Files and Filing. 

No. 50. Assembling Machine Tools, Part I. 

No. 51. Assembling Machine Tools, Part II. 

No. 57. Metal Spinning. 

No. 59. Machines, Tools and Methods of Auto¬ 
mobile Manufacture. 

No. 91. Operation of The Lathe, Part. I. 

No. 92. Operation of The Lathe, Part II. 

No. 93. Operation of Planer, Shaper and Slotter. 

No. 94. Operation of Drilling Machines. 

'To. 95. Operation of Boring Machines. 

No. 96. Operation of Milling Machines, Part I. 

No. 97. Operation of Milling Machines, Part II. 

No. 98. Operation of Grinding Machines. 

No. 116. Manufacture of Steel Balls. 

No. 120. Arbors and Work Holding Devices. 

No. 121, Machining Tapered and Spherical Sur¬ 
faces. 

No. 122. Broaching. 

No. 124. Cutting Lubricants. 

No. 128. Boring, Recessing and Multiple Turning 
Tools. 


TOOLMAKING 

No. 21. Measuring Tools. 

No. 31. Screw Thread Tools and Gages. 

No. 64. Gage Making and Lapping. 

No. 107. Drop Forging Dies and Die Sinking. 

No. 130. Gaging Tools and Methods. 

No. 135. Precision Locating and Dividing Methods. 

HARDENING AND TEMPERING 

No. 46. Hardening and Tempering. 

No. 63, Heat-treatment of Steel. 

JIGS AND FIXTURES 
No. 3. Drill Jigs. 

No. 4. Milling Fixtures. 

No. 41. Jigs and Fixtures, Part I. 

No. 42. Jigs and Fixtures, Part II. 

No. 43. Jigs and Fixtures, Part III. 

PUNCH AND DIE WORK 
No. 6. Punch and Die Work. 

No. 13. Blanking Dies. 

No. 26. Modern Punch and Die Construction. 

No. 126. Drawing, Forming and Bending Dies. 

No. 131. Die-making Practice. 

No. 132. Modern Blanking and Piercing Dies. 

AUTOMATIC SCREW MACHINE WORK 

No. 99, Operation of Brown & Sharpe Automatic 
Screw Machines. 


No. 100. Designing and Cutting Cams for the Au¬ 
tomatic Screw Machine. 

No. 101. Circular Forming and Cut-oif Tools for 
Automatic Screw Machines. 

No. 102. External Cutting Tools for Automatic 
Screw Machines. 

No. 103. Internal Cutting Tools for Automatic 
Screw Machines. 

No. 104. Threading Operations on Automatic 
Screw Machines. 

No. 105. Knurling Operations on Automatic Screw 
Machines. 

No. 106. Cross Drilling, Burring and Slotting Op¬ 
erations on Automatic Screw Machines, 

SHOP CALCULATIONS 

No. 18. Shop Arithmetic for the Machinist. 

No. 52. Advanced Shop Arithmetic for the Ma¬ 
chinist. 

No. 53. The Use of Logarithms—Complete Log¬ 
arithmic Tables. 

No. 54. Solution of Triangles, Part I. 

No. 55. Solution of Triangles, Part II. 

THEORETICAL MECHANICS 
No. 5. First Principles of Theoretical Mechanics. 
No. 19. Use of Formulas in Mechanics. 

GEARING 

No. 1. Worm Gearing. 

No. 15. Spur Gearing. 

No. 20. Spiral Gearing. 

No. 37. Bevel Gearing. 

No. 133. Hobs and Gear Hobbing. 

GENERAL MACHINE DESIGN 

No. 9. Designing and Cutting Cams. 

No. 11. Bearings. 

No. 17. Strength of Cylinders. 

No. 22. Calculation of Elements of Design. 

No. 24. Examples of Calculating Designs. 

No. 40. Flywheels. 

No. 56. Ball Bearings. 

No. 58. Helical and Elliptic Springs. 

No. 89. The Theory of Shrinkage and Forced Fits. 

MACHINE TOOL DESIGN 

No. 14. Details of Machine Tool Design. 

Np. 16. Machine Tool Drives. 

No‘. ijll. Lathe Bed Design. 

NtJ. 112. Machine Stops, Trips and Locking De¬ 
vices. 

CRANE DESIGN 

% 

No. 23. Theory of Crane Design. 

No. 47. Electric Overhead Cranes. 

No. 49. Girders for Electric Overhead Cranes. 
STEAM AND GAS ENGINES 

No. 65. Formulas and Constants for Gas Engine 
Design. 


SEE INSIDE BACK COVER FOR ADDITIONAL TITLES 


MACHINERY’S REFERENCE SERIES 

EACH NUMBER IS ONE UNIT IN A COMPLETE LIBRARY OF 
MACHINE DESIGN AND SHOP PRACTICE REVISED AND 
REPUBLISHED FROM MACHINERY 


NUMBER 141 

CASEHARDENING 


CONTENTS 

Introduction .. - 3 

Casehardening Practice in the Bicycle, Automobile and 
Allied Trades, by Robert H. Grant ----- 5 

Carbonization of Shafting - - - - - - - 19 

Casehardening Roller Bearing Parts, by E. F. Lake - 26 

New Casehardening Methods, by E. F. Lake - - - 36 


Copyright, 1914, The Industrial Press, Publishers of Machinery, 
140-148 Lafayette Street, New York City 




Other books in this series dealing with 
Heat-treatment of Steel and kindred 
subjects are as follows: 

No. 46 —Hardening and Tempering 
No. 62 —Hardness Testing of Metals 
No. 63 —Heat-treatment of Steel 
No. 117 —High-speed and Carbon Steel 
No. 118 —Properties of Alloy Steels 


"O 

A* ^ 





jf o, 

©CI.A388633 

\ ' ' • • I ' 

NOV 271914 

/ 


Machinery 

The Leading 
Mechanical Journal 

Machine Design 
Construction 

Shop Practice 
THE INDUSTRIAL PRESS 
140-148 Lafayette St. 

New York City 
51-52 Chancery Lane, London 













INTRODUCTION 


Casehardening is the process of increasing the carbon content of the 
surface of steel comparatively low in carbon, so that it can be hardened 
by the usual method of being heated to the hardening temperature 
and quenched in a cooling medium. The term casehardening, by it¬ 
self, implies the hardening of the surface or skin of an article, and 
in order to fully understand the process and its object, it is necessary 
to briefly consider the facts and laws upon which it is founded. Car¬ 
bon has a very great affinity for iron and combines with it at all 
temperatures above a faint red heat. Advantage was taken of this 
fact in the production of steel by cementation, an old process which 
consisted of rolling wrought iron into thin strips and then placing 
these in boxes with some material containing a fair proportion of 
carbon. These boxes were then heated to a very high temperature 
and the carbon was gradually absorbed by the iron. 

The process of casehardening is, in fact, only an improvement on 
this old cementation process used in times past for making steel 
from wrought iron. The steel is heated in packing boxes in the 
presence of a carbonaceous material and when the surface of the steel 
has absorbed enough carbon so that it will harden the same as high- 
carbon steel, it can be quenched in oil or water, according to the 
requirements. For many purposes, in machine work, articles are 
required which must have a perfectly hard surface and yet be of 
such internal structure that there is no chance of breaking them 
when in use. In many instances, this result can be obtained better 
by using casehardened mild steel than by using high-class crucible 
steel. For example, in making axles, cups, cones, and many similar 
parts for bicycles, it is extremely difficult to obtain perfect hardness 
combined with great resistance to torsional, shearing or bending 
stresses. For such purposes, nothing meets these requirements so 
well as do articles which have been casehardened. 

A great improvement has been made in casehardening processes 
during the last few years. The advance was begun with the develop¬ 
ment of the bicycle industry; and the necessity for casehardened 
parts of the highest quality in automobile manufacture has caused 
a still further improvement in this field. 

As an example of what has been accomplished by proper case- 
hardening methods, consider the transmission gearing of an auto¬ 
mobile. Who would think of throwing in the back-gears of a lathe 
or any other machine tool without first stopping the machine? In 
an automobile, however, this very thing is actually done dozens of 
times a day, by a person who gives little thought to what he is really 
doing. Yet the gears stand up under this treatment because of being 
manufactured of special steels developed during recent years and 
because of being heat-treated and casehardened by improved methods. 


4 


No. 141—CASEHARDENING 


There are a number of different questions that must be considered 
in order to obtain good results in casehardening. In the first place, 
the proper kind of steel to be used for various purposes must be care¬ 
fully selected. Another most essential thing is that the casehardening 
furnace must give a uniform heat. As oil and gas have to a great 
extent superseded coal as fuels for casehardening furnaces, the 
changes in furnace construction have, of late, been considerable. An¬ 
other item which must be given careful consideration is the box in 
which the material is packed, as well as the carbonaceous material 
itself used in packing the parts to be casehardened. Still another 
question to be dealt with is the method used for hardening the parts 
after they have been carbonized. 

Steel for Casehardened Parts 

As the casehardening process consists in adding carbon to the 
steel, a material must be used which will absorb carbon without 
necessitating overheating or burning. The effect of carbon on steel 
is, in general, it may be said, to make it dense, and the denser the 
steel the higher the heat necessary to open the pores through which it 
must absorb the carbon. A low-carbon steel containing, say, from 
0.15 to 0.20 per cent of carbon is, therefore, most suitable for case- 
hardening. It should also be borne in mind when selecting the ma¬ 
terial that the casehardening process does not eliminate any of the 
impurities ordinarily found in iron, such as sulphur, phosphorus, 
etc., and hence, a material as free as possible from these impurities 
should be selected; besides, the material should be perfectly sound 
and free from mechanical faults or weaknesses caused by overheating 
or improper working during the manufacturing processes. 

Both iron and mild steel have been employed as materials for case- 
hardening in the past; but this is the steel age, and iron has long 
passed its day. The steel employed should be prepared, selected and 
controlled from the beginning with the object of making it suitable 
for the final requirements. There are many points with relation to 
the selection of the proper steel, its composition and treatment, which 
can only be gained by long experience and a study of the require¬ 
ments, but, as a general rule, the low-carbon steel specified in the 
preceding paragraph will be found suitable for most purposes. 

In the following chapters will be given directions for casehardening, 
as published in Machinery by a number of authorities. It will be 
seen that opinions differ on certain points, and, therefore, the state¬ 
ments of each author have been given in full. The information given 
will necessarily overlap somewhat on this account, but, on the other 
hand, a complete and authoritative presentation of the whole subject 
has been made possible. 


CHAPTER I 


CASEHARDENING PRACTICE IN THE BICYCLE, 
AUTOMOBILE AND ALLIED TRADES 

In building or constructing a furnace for casehardening, the size 
of the work to be hardened should be the first consideration. It is 
far better to use a small furnace with a small box whenever pos¬ 
sible. If the work varies in size, different sizes of furnaces may be 
used. Small furnaces require less fuel, and small work must be 
placed in small boxes as otherwise the pieces packed near the sides 
will be overheated while those in the center will not reach the re¬ 
quired temperature. The furnaces should be made right- and left- 
hand so that they can be placed close together. Thick walls should 
be used to retain the heat. These walls should be supported by a 
substantial concrete foundation, so that they will retain their position 
and shape, even when subjected to a high heat. Large flues should be 
provided to carry away the smoke and gases. 

The furnace should also be so constructed that as much as pos¬ 
sible of the heat of the combustion gases may be extracted before they 
are discharged. The flues and all parts of the furnace should be 
easily accessible, and a door, the full width of the oven, should be 
provided so that the tiles can be taken out and the flues cleaned. 
A pressure blower with a light oil should be used with all the pipes 
accessible and placed, preferably, above the furnace. If, however, they 
are placed below ground, they should be arranged in compartments 
which can be easily reached if repairs are required. 

The blower pipes should be run through the furnace so as to pre¬ 
heat the air used; if cold air is used directly it will reduce the heat 
in the furnace. The furnace fronts should be made in several parts 
to prevent cracking, with the door properly balanced and lined. A 
shelf should be provided, projecting at the front, for holding the boxes 
when they are taken out or put into the furnace. The smokestack 
should be made of sufficient height to produce a good draft. 

Burners should be placed both at the front and rear of the oven 
and should be arranged in separate compartments, so that the heat 
will be uniform in the oven. The hot gases will then pass over the 
top of the compartment wall and strike the boxes on the top, after 
which they pass out through small openings in the corner of the 
furnace. They then take a zigzag course under the tiles and pass 
from there through a flue to the rear of the furnace. A large conduit 
should be provided just below the ground which will catch all the 
soot. This conduit should be provided with iron covers which can 
easily be taken off to remove the accumulation of soot. 

The furnace should not be heated too quickly, as this is apt to 
crack the brickwork. The cooling should also be done gradually. 


6 


No. 141—CASEHARDENING 


After the work has been taken out and the heat shut off for the day, 
all the dampers should he closed to hold the heat. In this way the 
furnace will cool slowly and cracking or bulging out of shape will be 
prevented. In addition, it will be easier to heat the furnace the next 
morning, as it will have retained some of the heat. 

When work is to be annealed, it should be placed in the furnace 
after the work to be hardened has been removed, and then the furnace 
brought to the proper heat. The material to be annealed can then 



remain in the furnace until the next morning with the furnace closed 
up and the burners turned off. 

A light oil should be used. It should have a high heating value 
and be comparatively free from carbon deposits. 

Boxes for Casehardening 

The boxes for casehardening should not be made larger than neces- 
ray for the class of work being handled. They should be made from 
a malleable iron, as ordinary cast-iron boxes are not suitable on ac¬ 
count of the fact that they are very porous and absorb the carbon 
of the carbonizing material. The boxes should also be provided with 
feet, as shown in Fig. 1, so that the heat can circulate all around 
them. The covers should be provided with ribs on the top to prevent 
excessive warping, and the sides should be ribbed so that a fork or 










































































CASEHARDENING 


7 


grapple iron, as shown in the upper part of Fig. 2, can be used for 
handling the boxes. The sides of the boxes should taper slightly 
towards the bottom so that the contents can be easily dumped out 
of them; they are also easier to cast when made in that way. 

When very large boxes are required, they should, if possible, be 
provided with a hole through the center so that the heat can reach 
the contents from the inside, as well as from the outside. A box 
of this kind is shown in Fig. 3. For long work, such as shafts, tubing, 
etc., a wrought-iron pipe with a cap on each end provides an ideal 
box. 

Local Hardening- 

In many cases it is essential that the piece of work be hardened 
at a certain place and that other parts be left soft. There are three 
ways in which this can be accomplished: First, by copper-plating 



Fig. 2. Grapple Iron or Fork used for Handling Casehardening Boxes 
and Truck for Handling Heavy Boxes 


and enameling; second, by covering the part which is not to be 
hardened by fireclay; and third, by using a bushing or collar to cover 
the part to be left soft. 

In the first case the article should be painted with enamel where 
it is to be hardened, the enamel being baked after having been applied. 
The remainder of the piece that is to be left soft is copper-plated. In 
the second case, if the article to be hardened has a recess, such as 
a hole, slot, etc., this may be filled with clay. The third method is 
used when a shaft, for example, is only to be left soft for a short 
distance. A collar is then placed on the shaft, and this provides the 
easiest and least expensive means for accomplishing the purpose. 














8 


No. 141—CASEHARDENING 


In the case where enamel and copper-plating is used, the enamel 
will burn away and allow the surface covered by it to absorb carbon 
and, hence, to be hardened, whereas the copper will stand a very 
high heat and prevent hardening of those portions that are covered 
by it. If the copper is burned off, it is an indication that the work 
has been overheated. The clay prevents the hardening of a portion 
of the work in the same way as does the copper. It is also of ad¬ 
vantage when dipping the work, as it prevents the formation of steam 
pockets which are apt to warp or distort the piece. When a sleeve 



Machinery 

Fig:. 3. Large Circular Box with Hole in Center for the Circulation 
of the Gases of Combustion 

or collar is used, this should be made about one-half inch longer 
than the part which is to be left soft, so as to prevent carbonization 
near the ends of the collar. 

Packing: for Hardening- 

The packing room should, if possible, be separate from the room 
containing the furnaces, so that the packing can be done without the 
discomfort of the heat and dust. Tables on wheels, or trucks, provided 
with shelves of the same height as the shelf in front of the furnace 
and large enough to hold the required number of boxes for one fur- 



































CASEHARDENING 


9 


nace, should be provided, so that the packed boxes can be easily 
moved to the furnace and quickly placed in it. The work to be hard¬ 
ened should be classified according to its size and the percentage of 
carbon required, as it will take a higher heat for larger work, as well 
as for pieces which are required to absorb a higher percentage of 
carbon. 

There are a great many different kinds of hardening materials, 
but the old-fashioned method of using ground bone can always be 
relied upon to give satisfactory results. During the last few years, 
however, the use of bone in various manufacturers has increased so 
that the price of ground bone for casehardening purposes is almost 
prohibitive. Leather has become very extensively used for this pur¬ 
pose, it being first burned and then ground and graded. 

A mixing bin is a great advantage in connection with the handling 
of the casehardening material. Some partly used bone and some 
new is then used to make a mixture suitable for the size of the pieces 
to be hardened. Large pieces require a richer material than smaller 
ones, as during the higher heat required for the larger pieces and the 
longer application of the heat, more of the carbonizing material will 
burn away. 

When packing a box, first put a layer of the casehardening material 
on the bottom, the thickness of this layer depending upon the size 
of the pieces to be hardened. If the articles are heavy, they do not 
require such great care in packing, but if they are thin or long, or 
have a peculiar shape, greater care is required. It has frequently been 
stated that one piece should never be permitted to touch another when 
packing, but it has been found that this precaution is not necessary. 
If a box is properly sealed, the parts can touch each other without 
injury. Thin long pieces should, if possible, be placed in an upright 
position to prevent their sagging out of shape. Between each layer 
of pieces, casehardening material is packed according to the size of 
the pieces to be hardened. It has been found from experience that 
if there is not enough of the carbonizing material in the box, the work 
is liable to have soft spots. 

About two inches from the top of the box, sheet steel strips about 
1/16 inch thick should be laid and these should be covered with a 
layer of about one inch or more of powdered charcoal. Then the 
cover is placed on the box and the edges are sealed with fireclay. If 
there is any doubt about the length of time required for heating the 
pieces to obtain a certain depth of case, wire a couple of pieces to¬ 
gether, allowing the wire to project out of the box. These pieces can 
then be taken out quickly and hardened, and, in this way, it can be 
ascertained whether the parts have been sufficiently carbonized. In 
casehardening very small work, it is advisable to wire the pieces 
together so that they can be taken out of the box at once; otherwise, 
they would have to be picked out with small tongs, as it is impossible 
to sift very small work in a screen because the mesh would have to 
be so fine that it would take a long time to do the sifting and the work 
would become too cold for hardening. If it is desirable to color the 


10 No. 141—CASEHARDENING 

work, from one-third to one-half of the carbonizing material should be 
burnt leather. 

The boxes should never be put into the furnace under a high heat, 
but should be placed in it when its temperature is from 800 to 900 
degrees F. Then the heat should be slowly brought up to from 1500 
to 1800 degrees F. In placing the boxes in the furnace, great care 
should be taken that the hot gases have an opportunity to circulate 
all around them. A pyrometer should be put in some convenient 
place and properly wired so that the heat in the furnace can be readily 
ascertained at any time. If there is a great deal of night work to be 
done, a recording pyrometer should be used as it gives the man in 
charge a record of the heats during the night. 

By the aid of the pyrometer it has been found that it is necessary 
to have an expansion tank in order to get a constant air pressure, 
otherwise the pulsation from the blower will affect the heat in the 
furnace. This expansion tank should be situated so that the blower 
is connected directly with one end while the discharge pipe is con¬ 
nected at the opposite end. This will then act as a reservoir, produc¬ 
ing a constant pressure. When oil is used for the heating, it is pre¬ 
ferable to pump it from the storage tank in the ground to a stand 
pipe, which will insure a constant flow of the oil. The intermittent 
action of the pump, should the oil be used directly as it comes from 
it, is objectionable. There is also another advantage, in case the pump 
should have to be shut down on account of break-down. In that case, 
the furnaces could still continue to operate, as the stand pipe should 
hold a supply of oil sufficient for several hours. At night and on holi¬ 
days the oil should be drained back into the storage tank in order to 
minimize the danger incident to its use. 

The supply pipe for the air should come from the outside and should 
be so arranged that the air passes through a fine wire netting, so as 
to prevent foreign substances from entering the blower. 

On the outside of each furnace a card should be placed telling the 
kind of work that is in the furnace, when the work was put in, the 
heat required for it, and when it is to be removed. These cards can be 
kept as a record which Will be of value when comparison is made with 
the depth of case obtained under any specific conditions. 

Carbonizing, Reheating and Hardening 

The heat required for casehardening is a great deal higher than 
that required for ordinary hardening. If, for example, the material 
to be casehardened was heated only to 1375 degrees F., which would 
be sufficient for the hardening of ordinary tool steel, the result would 
be very unsatisfactory. In fact, there would be no result at all. Small 
parts must be heated to at least 1575 degrees F., in which case suf¬ 
ficient depth of carbonized surface will be obtained in from six to 
eight hours. The time recorded as the correct one for casehardening 
should be taken from the time the boxes are heated clear through. 

The correct way in which to caseharden is first to carbonize the 
material and then to allow the boxes to cool down with the work in 


CASEHARDENING 


11 


them, after which they are reheated and hardened in water. The re¬ 
heating refines the grain of the steel and prevents the formation of 
a distinct line between the outer hardened case and the soft core. If 
there is a distinct line between these two sections, the case is liable 
to flake off when the hardened part is subjected to severe stresses. 

A still more refined method of casehardening is to repack the work, 
after it has been carbonized, in old bone, and after heating for two 
or three hours take it out and dip the pieces in the hardening tank 
directly as they come from the boxes. This will produce a very fine 
grain and in many cases prevent warping. If the work is large and 
it is required to toughen the inner core, it should be reheated to a 
higher heat than otherwise; then, after dipping, reheat again to 1500 
or 1600 degrees P. according to the size of the work, and redip. 

However, if the work to be hardened consists of bolts, nuts, screws, 
etc., it is satisfactory to dump them into water directly from the fur¬ 
naces, without any reheating. A regular iron wheelbarrow with two 



Fig. 4. Mandrel used when Hardening Collars, etc., on the Outside 


pieces of flat iron placed across it lengthwise should be provided. On 
top of these bars is placed a sieve made from %-inch wire with %-inch 
mesh, about 18 inches square by 6 inches deep. This sieve should have 
a handle 6. feet long and % inch in diameter. The boxes are emptied 
into this sieve, and after sifting, the heated material is dumped into a 
tank of cold water which should be of sufficient size to prevent the 
water from heating too quickly. Care should also be taken in empty¬ 
ing the contents of the boxes into the water that they are not all 
dumped in one place, but scattered about in the tank. A constant 
flow of water should be available while the work is being hardened. 
The work should under no circumstances be removed from the furnace 
until the heat has been lowered, as the steel should be treated as tool 
steel after it is carbonized, and it would be injurious to the steel to 
harden it at the high carbonizing heat. 

Gears and other parts which should be tough, but not glass hard, 
should preferably be hardened in an oil bath. There is then less 
liability of warping the work, and the hardened product will stand 
shocks and severe stresses without breakage. Cotton-seed oil is the 
best hardening medium to be used in this case. 

After the work has been properly carbonized, the next operation in 
the case of all parts, except those mentioned as exceptions above, is 
to reheat. This may be done either as already explained, or it may 












12 


No. 141—CASEHARDENING 


be done in a regular muffle gas furnace in which the work can be put 
in rows on the tile. In this way the work can be heated very slowly, 
a new piece being put into the furnace to take the place of each piece 
as it is removed. Collars, etc., which are required to be hardened 
on the outside, but ought to be left soft on the inside, should be hard¬ 
ened on a mandrel, such as shown in Fig. 4, the diameter of the 
mandrel being from 0.001 to 0.003 inch smaller than the hole in the 
piece to be hardened. If the inside of the piece only needs to be 
hardened and the outside should be left soft, a cup-shaped holder, such 
as shown by the dash-dotted lines in Fig. 5, may be used. In this case 
the work will harden at B while it is left soft at A and G. 

The hardening tank should be about 30 inches in diameter and 36 
inches deep and have a constant flow of water from a pipe in the 
center about 6 inches below the surface. 

Straightening 1 the Work after Hardening 

On account of the manner in which steel is rolled, drawn or forged, 
the density varies in different parts of the steel, and no matter whether 



the material is heat-treated or not, it will warp more or less when 
hardened. It is, therefore, necessary to provide apparatus for 
straightening the work. In straightening, it is necessary to bend the 
work about twice as much as would be required to merely keep it 
straight while the pressure is applied, as, on account of its elasticity, 
it will have a tendency to work back to its original form. Small 
rollers and shafts can best be straightened in a vise by having a three- 
point contact on the jaws. For large diameters a special straightener 
will be required. A surface plate placed to the height of a man’s eye, 
and at a slight angle towards the light, provides the easiest means for 
testing work of this character while being straightened. 

When there is a large quantity of rings to be straightened or trued 
up, a surface plate can be readily rigged up in the following manner: 
A solid strap is provided on one side and a compound lever on the 
other, adjustable to any place along the plate by means of a slot in 
the latter. By a slight movement of the lever the ring can be trued 
up. An indicator should be placed at the front of the plate so that the 
operator can try a ring to see at which points the ring is out, and 
also the amount necessary for making it round. In straightening 











CASEHARDENING 


13 


washers or flat pieces of any kind, the hydraulic press provides the 
best possible means. It might be well to mention that washers or flat 
pieces should be ground by taking a small amount off each side 
alternately, as, otherwise, they will return to their original warped 
shape. Another precaution, relating to the grinding of cylindrical 
surfaces, is to use a copious supply of water, as otherwise the heat of 
the grinding operation will draw the surface, producing soft spots. 
These will appear to have been caused by improper casehardenihg, but 
as a matter of fact, they are wholly produced during the grinding 
operation. 

Standard Practice for Small Work 

In dealing with the subject of casehardening in a paper before the 
Cycle Engineers’ Institute, Mr. D. Flather covered partially the same 
ground as has been covered in the preceding pages. A number of his 
recommendations will, however, prove of value, and are given in the 
following pages. He states that the furnace should be so constructed 
as to be capable of being raised to a full orange heat (1830 degrees F.), 
and maintained at that heat with great regularity. It should be so con¬ 
structed that neither the fuel nor the direct flame can come in contact 
with the charge. The flames should uniformly impinge on the sides 
and roof of the muffle in such a manner as to raise them to a high 
temperature, thus heating the contents of the muffle by radiated and 
not by direct heat. A furnace designed on this principle not only 
gives the best result but is also most economical in the matter of fuel. 
The muffle chamber and flues must, of course, but constructed of fire¬ 
brick, and the doors should fit closely and also be lined with firebrick. 
It is important that there should be a small peep-hole in the door, with 
a cover plate; a hole 1 y 2 inch in diameter is quite large enough. This 
latter is a most important detail, as it provides against the need of 
opening the doors in order to judge the heat, and is indeed the most 
accurate means of estimating the temperature by the eye. The fur¬ 
nace must be fitted with a reliable damped plate or other effectual 
means of controlling the draft. 

Hardening pots are made in both cast and wrought iron, the former 
being cheaper in first cost, but the latter bear reheating so many 
times that they are cheaper in the end. The pots should not be of 
too large dimensions, or there is great risk of articles in the middle 
of a charge not being carbonized to a sufficient depth. No pot should 
be above 18 by 12 by 11 inches for such parts as cycle axles, pedal pins, 
and the like; while for small articles like cups, cones, etc., 12 by 10 by 
8 inches is large enough. The pots should each have a plate-lid fitting 
closely inside. 

The carbonizers in general use at the present day are animal char¬ 
coal, bones, and one or two other compositions sold under various 
names, consisting of mixtures of carbonaceous matter and certain cyan¬ 
ides or nitrates. For very slight hardening, cyanides alone are still 
found very useful, but no great depth of casing is ever attempted with 
these. Theoretically, the perfect carbonizer should be a simple and 


14 


No. 141—CASBHARDENING 


pure form of carbon, and good charred leather gives the most certain 
and satisfactory results. Care should be taken to avoid poorly charred 
leather or that made from old boots, belting, etc. 

As clay must be used for a luting around the pot lid, and is also 
frequently used for stopping off portions to be left soft, it is important 
to see that a good clay is used, and that it is free from grease. Clay 
contaminated with grease in any way will cause irregularity in the 
product. 

Reheating- Muffles 

As all casehardened articles have to be reheated before quenching, 
it is important that a suitable furnace should be employed for the pur¬ 
pose. It is not advisable that the reheating should be done in the 
casehardening muffle, unless it is run specially for the purpose and 
at a lower heat. If possible a small gas muffle should be used for re¬ 
heating, and indeed for all hardening work. A properly constructed 
gas muffle can be regulated with great exactness, and this is very im¬ 
portant in all hardening. 

Packing- the Muffles 

The carbonizer having been thoroughly dried and reduced to a fine 
powder, a layer of not less than iy 2 inch in depth is placed in the 
hardening pot and well pressed down. Upon this are placed the 
articles to be hardened. Care must be taken to leave sufficient space 
all around each piece to prevent its touching the others or the walls 
of the pot; a space of 1 y 2 inch should be sufficient. Another layer of 
carbonizer is then put in and well pressed down, taking care not to 
displace any of the articles already packed, continuing until the pot is 
nearly full, and then finishing off with another layer of 1 y 2 inch at 
the top. The object in view must be to make the contents of the pot 
as compact as possible, consistent with a sufficiency of carbonizer in 
contact with the articles. The more solidly a pot is packed the more 
complete is the exclusion of air. The lid is then put on, and the joint 
all around well luted with clay. By the time the proper number of 
pots have been filled, the furnace must have been raised steadily to the 
full working heat. 

Furnace Heat 

The proper heat for casehardening is about 1800 degrees F., or a 
full orange heat and this should be maintained with great regularity 
throughout the operation. The length of time occupied in carbonizing 
is regulated by the depth of casing required, and indirectly by the 
dimensions of the article. At the close of the carbonizing period the 
pot is withdrawn from the furnace and placed in a dry place, where it 
is allowed to become quite cold. It is then opened, the articles taken 
out and brushed over to remove all adhering matter. If the pot has 
been properly packed and luted up, the articles should be quite white, 
or at least have only a slight film or bloom of a deep blue color; the 
denser and more inclined to redness is the surface, the more imperfect 
has been the packing and sealing of the pot. 


CASEHARDENING 


15 


Reheating- and Hardening 

The carbonized articles are now placed in a muffle furnace and 
steadily raised to a good cherry red (1470 dtegrees F.), and then 
quenched in cold or tepid water or oil, according to the purpose of the 
articles required. They should remain in the cooling liquid until they 
are quite cold right through the body of the metal, thus completing the 
process. 

Although the proper temperature for casehardening is about 1830 
degrees F., this temperature may be modified to suit the purpose in 
view. The absorption of the carbon commences when the steel reaches 
a low cherry-red heat (1300 degrees F.); it begins, of course, at the 
outer surface and gradually spreads until the whole of the steel is 
carbonized. The length of time this requires depends upon the thick¬ 
ness of the metal being treated. The percentage of carbon absorbed is 
governed by the temperature, and although the increase of carbon is 
not in uniform proportion to the rising temperature throughout, it is 
perhaps sufficient for our present purpose to note that at 1300 degrees 
F., iron, if completely saturated, can contain no more than about 0.50 
per cent carbon; at 1650 degrees F., about 1.5 per cent carbon; and 
at 2000 degrees F., about 2.5 per cent. These results, however, are only 
obtainable when the whole section of the iron has received all the car¬ 
bon it is capable of absorbing at the given temperature, and is there¬ 
fore in a state of equilibrium. From this it will be seen that if the 
process is stopped before the action is complete, the central parts of 
the iron must contain less carbon than the outside, and upon this fact 
the process of casehardening is founded. 

If we take two pieces of % inch diameter round mild steel, and 
heat one of them with a carbonizer at a cherry-red heat, and the other 
at a bright orange heat, for six hours, the first will be cased to a depth 
of about 1/32 inch, and the other to a depth of nearly 1/16 inch, while 
the amount of carbon taken up will be about 0.50 and 0.80 per cent re¬ 
spectively; so that, so far as regards the hardness of the skin, the 
piece carbonized at the higher temperature gives the best result. From 
this we learn that a temperature of 1830 degrees F. will give us suffi¬ 
cient hardness of case. 

We have next to find which temperature has the least harmful effect 
on the mild steel core, and this can best be found by heating pieces 
of the mild steel at varying temperatures at and above the selected 
one for the same length of time, using lime or other inert substance in 
the pot instead of a carbonizing material, and afterward reheating and 
quenching in water. Suppose, for example, we take three pieces, heat¬ 
ing at 1830, 2370 and 2730 degrees F., or full orange, white and bright 
white respectively. We shall find that those at 2370 and 2730 degrees 
break very short and have lost nearly all their original tenacity, 
while that at 1830 degrees appears tougher and altogether stronger 
than before. 

Having arrived at a knowledge of the right temperature, it remains 
now to inquire as to the length of time requisite to yield a sufficient 


16 


No. 141—CASEHARDENING 


depth of case. At a full orange heat a bracket cup of ordinary di¬ 
mensions should in two hours be hardened 1/32 inch deep, and a 
bracket axle 11/16 inch diameter in 6 hours would have a case 1/16 
inch deep. Prom this it will be seen that the speed of penetration is 
not in exact proportion to the time of heating. 

Results of Hardening- without Reheating- 

We now arrive at that part of the process where a most important 
improvement has been made— i. e., the final hardening by quenching 
in water. It formerly was customary at the end of the carbonizing 
period to open the pot and fling the contents headlong into a tank 
of cold water. Here and there some of the more careful workers took 
each article separately, but direct from the pot, and plunged it into 
water. These latter obtained better results, but even they had a great 
deal of trouble in the way of breakages and want of regular hardness. 
Finding that axles taken singly from the pot and quenched were better 
than those quenched in bulk, and that if allowed to cool down to 
cherry red they were better still, an application of the old rule to 
harden on a rising heat led to the now established principle of allow¬ 
ing the pot and its contents to become quite cold, afterward reheating 
to cherry red and quenching with water. By this means we obtain 
a case of great hardness with a very tough core—that is, of course, 
provided a suitable steel is employed. 

To understand the reason of this improved method of working we 
must remember that the exterior of the steel is now of about 0.80 
per cent carbon, and that steel of all kinds raised to and maintained 
at the high temperature employed for casehardening will, unless sub¬ 
jected to mechanical work, show evidence of overheating, being very 
brittle and liable to easy fracture; and though quenched in water, 
and consequently hardened, the metal has little or no cohesion and 
readily wears away. Steel so hardened breaks with a very coarse 
crystalline fracture, in which the limits of the case are badly defined. 
It is known that when steel is gradually heated there is a certain 
point at which a great molecular change takes place, and that perfect 
hardness can only be obtained by quenching at this critical point. 
If quenching takes place below the critical temperature, the steel is 
not sufficiently hard; if above, though full hardness may be obtained, 
strength and tenacity are lost in part or completely, according as the 
critical temperature is exceeded by much or by little. This critical 
point lies between 1380 and 1470 degrees F., or cherry-red color heat. 
It may be asked why it is not sufficient, when taking the article out 
of the pot, to allow it to cool down to cherry red and then quench it. 
To this the answer is that the high temperature has already created 
a coarsely crystalline condition in the steel, and that until it has 
become quite cold and has again been heated up to the critical tem¬ 
perature, a suitable molecular condition cannot be obtained. When 
steel is cooled, whether slowly or not, it bears in its structure a con¬ 
dition representative of the highest heat it was last subjected to. 


CASEHARDENING 


17 


Casehardening Practice of Pennsylvania Railroad Shops 

It may be of interest to note the casehardening practice followed 
by the Altoona shops of the Pennsylvania Railroad Company. The 
compound for casehardening is made from 11 pounds prussiate of 
potash, 30 pounds sal soda, 20 pounds coarse salt, and 6 bushels 
powdered charcoal (hickory preferred). These ingredients are mixed 
thoroughly, using 30 quarts of water in mixing. The following method 
is pursued in packing the material to be casehardened. The bottom 
of the box is covered to a depth of 2 inches with the compound. The 
parts to be hardened are placed solidly so that the compound is in 
contact with the bottom surface of the part, care being taken that the 
work does not touch the sides of the box or other pieces. After the 
first layer of the material is placed, it is covered on all sides and 
on top with the compound and solidly packed. After the first course 
is packed the process is repeated, care being taken to have a sufficient 
amount of compound between every course. There should be not less 
than 2 inches of compound on the top of the last course. Then the 
lid is thoroughly sealed with a luting of fireclay. 

In the furnace the box rests on rollers to allow the flames to pass 
under it. When the material has been in the furnace a sufficient 
length of time, the box is withdrawn to a trestle flush with the floor 
of the furnace and parallel with and close to a water tank, after 
which the material is removed from the box and plunged into the 
water. This method makes it possible to obtain a depth of case on 
large material of from 1/16 to 5/32 inch in 14 hours, and of about 1/16 
inch on bushings and small parts in from 2% to 3 hours. All parts 
to be casehardened are thoroughly cleaned so as to be free from oil 
or grease. 

American Society for Testing- Materials Standard 
Casehardening- Practice 

The following practice for heat-treating casehardened carbon steel 
has been recommended and adopted by the American Society for 
Testing Materials. 

1. When hardness of case only is desired and lack of toughness 
or even brittleness is unimportant, the carburized objects may be 
quenched from the carburizing temperature, as for instance, by empty¬ 
ing the contents of the boxes into cold water or oil. Both the core 
and the case are then coarsely crystalline. 

2. In order to reduce the hardening stresses and to decrease the 
danger of distortion and cracking in the quenching bath, the objects 
may be removed from the box and allowed to cool before quenching 
to a temperature slightly exceeding the critical range of the case, 
namely, 800 to 825 degrees C. Both the core and case remain coarsely 
crystalline. 

3. To refine the case and increase its toughness, the carburized 
objects should be allowed to cool slowly in the carburizing box within 
the furnace or outside to 650 degrees C., or below, and should then 
be reheated to a temperature slightly exceeding the lower critical 


18 


No. 141—CASEHARDENING 


point of the case (in the majority of instances a temperature varying 
in accordance with the carbon content and thickness of the case be¬ 
tween 775 and 825 degrees C., will be suitable), and quenched in 
water, or, for greater toughness but less hardness, in oil. The objects 
should be removed from the quenching bath before their temperature 
has fallen below 100 degrees C. This treatment is more especially 
to be recommended when the carburizing temperature has not ex¬ 
ceeded 900 degrees C. It refines the case but not the core. 

4. To refine both the core and the case and to increase • their 
toughness, the objects should be allowed to cool slowly from the 
carburizing temperature to 650 degrees C. or below and should then 
be ( a ) reheated to a temperature exceeding the critical point of the 
core, which will generally be from 900 to 950 degrees C., followed by 
quenching in water or in oil; and (&) before they have cooled below 
100 degrees C., they should be reheated to a temperature slightly 
exceeding the lower critical point of the case (in the majority of in¬ 
stances a temperature varying in accordance with the carbon content 
and thickness of the case between 775 and 825 degrees C. will be 
suitable), and again quenched in water or oil. 

The objects should be removed from the quenching bath before 
they have cooled below 100 degrees C., in order to lessen the danger 
of cracking, and they should be placed in the reheating furnace while 
still at a temperature of at least 100 degrees C., likewise to lessen the 
danger of cracking, it being inadvisable (a) to allow steel to cool 
completely in the quenching bath and (6) to place hardened steel 
in a hot furnace. Obviously, if the furnace is cold the hardened steel 
may likewise be cold when placed in it for reheating. 

5. In order to reduce the hardening stresses created by quenching, 
the objects, as a final treatment, may be tempered by reheating them 
to a temperature not exceeding 200 degrees C. 


CHAPTER II 


CARBONIZATION OF SHAFTING 

The increasing use of anti-friction bearings in various forms, as well 
as other developments in the construction of machinery, has made 
necessary the use of harder and better surfaces for shafts than has 
heretofore been considered good practice. Before entering into any 
detailed discussion of this subject, we should first have some under¬ 
standing of what the problem really is, expressed, if possible, in 
concrete figures. Let us take a piece of soft steel which has been 
turned and ground to a definite size. We find its hardness, as meas¬ 
ured by the Shore scleroscope, to be somewhere between 15 and 25, 
whereas with a piece of cold-rolled shafting, we obtain 30. Alloy 
steel of suitable analysis and properly treated will give 60, and if we 
take still another piece of material, carbonize it and follow this by 
suitable heat-treatment, we can obtain 80, as shown by the scale of 
the instrument. Therefore, we can see from these approximate figures, 
that the method and material to be used is largely a matter of the 
specific result desired and, obviously, it is impossible to utilize any 
one method or material for all requirements. The information given 
in the following is based on a paper read by Mr. J. G. Weiss, before 
the National Machine Tool Builders’ Association. 

Aside from the condition of the surface, there are other important 
considerations. Taking the elastic limit of the material as a measure 
of its load-carrying capacity, we find 40,000 pounds per square inch 
an average result for soft steel and 170,000 pounds not excessive for 
properly heat-treated alloy steels. Hence, we see the possibility, in 
alloy steels, of not only obtaining the required surface, but, at the 
same time, materially increasing the factor of safety of the shaft, or 
using a smaller shaft with the same factor, as may be preferable. In 
the case of carbonized shafts, there is no material increase in the 
elastic limit, the improvement being entirely a matter of surface 
condition. 

If the surface requirements are represented by a scleroscope reading 
greater than approximately 60 to 65, the problem must be approached 
from the standpoint of carbonization, unless we are willing to use 
expensive alloy steels which, in this discussion, are considered neither 
possible from the standpoint of first cost nor a necessity. Carboniza¬ 
tion gives ideal results as to surface conditions but no increase in 
the elastic limit. On the other hand, if a scleroscope reading of 60 
to 65, or less, is considered to be a satisfactory standard, it is feasible 
to use many alloy steels at low cost which, when of the proper analysis 
and suitably heat-treated, will not only give equally good results as 
to surface conditions, but a material increase in strength as well. 
The problem then is naturally divided into two general divisions 
each requiring a different discussion. First, carbonized shafts having 


20 


No. 141—CASEHARDENING 


a surface hardness greater than 60 to 65 scleroscope reading; second 
heat-treated shafts having less hardness than the figures stated. The' 
first of these conditions only will be dealt with here. 

Carbonized Shafts 

Perhaps no branch of the heat-treatment of steel has been more 
thoroughly investigated than that of carbonization, but the results 
show conclusively that there is no standard American practice, either 
in regard to methods or material. This is doubtless due to variation 
in local conditions. For instance, one who carbonizes arbors or shaft¬ 
ing is not so much concerned about the toughness of the core as one 
who treats pieces of thin cross-section. Again, one so-called au¬ 
thority will insist that the transition from case to core should be 
gradual with no sharp line of demarkation, whereas one who is car¬ 
bonizing very thin sections or shells knows from experience that the 
case must, necessarily, be distinctly defined and concentrated in order 
to use to the best advantage the small space allowable to obtain such 
conflicting properties as toughness and hardness. 

Where the number of parts requiring carbonization is comparatively 
small, a high-speed carbonizer whose strength is spent on the first 
run may prove satisfactory, but if the number of parts runs up into 
the thousands daily, a more economical material would be adopted. 
If our problem is that of a few pieces a week, the irritating effect 
of prussiate of potash is not particularly objectionable, while with a 
condition involving many thousand pieces daily, in all extremes of 
weather, its effects are more in evidence. Thus, the authority who 
endeavors to lay down a hard and fast rule is treading on dangerous 
ground. Bearing in mind, therefore, the wide divergence of condi¬ 
tions, even when limiting ourselves to shafts, let us first consider the 
best material for our specific purposes. 

Steel for Carbonizing 

In considering the steel to be used naturally the carbon content 
is the point at issue. A material varying from 0.15 to 0.20 per cent 
carbon is the most satisfactory from every viewpoint. Many au¬ 
thorities recommend a carbon content as high as 0.27 per cent, but 
others claim that it has not been found possible to obtain uniform 
and satisfactory results, in a large way, with such material. Irre¬ 
spective of the particular analysis that may be decided upon, the 
matter of uniformity is of even greater importance. No matter what 
the source of supply may be, it is advisable to analyze ten per cent 
of the material received, in order to avoid irregularities which sooner 
or later develop. 

Carbonizing Mixtures 

Most of the carbonizing compounds upon the market contain simple 
ingredients which can be mixed readily upon the premises, although 
some are almost beyond the realm of chemical analysis. In fairness 
it may be stated that a few of them give very good results, but the 
price is generally set in proportion. For all-around purposes, those 


CASEHARDENING 


21 


which can be mixed easily upon the premises are best. If this method 
he adopted, the metallurgist in charge can superintend the entire 
grinding and mixing process and keep the quality of each ingredient 
up to a standard. The two following mixtures, which for convenience 
are designated “A” and “B,” are easily prepared, reasonable in cost 
and have given satisfaction in the treatment of several million pieces 
of the most exacting requirements. 


MIXTURE A 

Per Cent 


Raw bone. 35 

Bone black. 27 

Charred leather. 11 

Wood charcoal. 27 


MIXTURE B 

Per Cent 


Potassium ferro cyanide. 5 

Sal soda. 14 

Coarse salt. 9 

Powdered wood charcoal. 72 


100 100 


The characteristics to be considered in a carbonizing mixture are: 
Hardness imparted to the work; rate of penetration; cost per pound 
and renewal cost; ease of manipulation in grinding, mixing, pack¬ 
ing, etc. The following table shows mixtures “A” and “B” compared 
in the first three respects, with one of the best carbonizers on the 
market, which for convenience is designated as mixture “C.” 


Material 

Scleroscope 

Reading 

Time to 
Penetrate 

5/64 inch 

Cost Per 
Pound, 
Cents 

Per Cent of 
New Material 
Required for 
Renewal 

“A” 

70 

13 hours 

2.6 

55 

“B” 

75 

12 hours 

1.3 

30 

“C” 

75 

14 hours 

2.5 

80 


The hardness was determined by using properly carbonized heat- 
treated and polished specimens and naturally varied somewhat, the 
values given in the table being averages of several readings. The 
time to penetrate to a given depth was obtained on specimens of 
shafting one inch in diameter. The carbonizing process was similar 
to that recommended in the following. By referring to the preceding 
table, it will be seen that mixture “B” shows a gain of about 14 per 
cent over mixture “C” and about 8 per cent over “A,” in respect to 
speed of penetration. In initial cost, material “B” is the cheapest, 
while “A” and “C” are about the same. The quantity of fresh material 
which must be added to that which has already been in the furnace, 
to restore it to the desired strength, shows a marked advantage in 
favor of “B.” Moreover, material “C” was practically useless after 
one heat. While cost is an item of minor importance, and quality is 
cheap at any price, if the other properties are balanced, cost might 
judiciously be considered in selecting carbonizing mixtures; hence, 
this data was added for the sake of completeness. 

Packing: Shafts to be Casehardened 
If the shafts are not too long, they should preferably be packed in 
pots standing on their ends, but if the length does not permit of this 
arrangement, they may be packed in rectangular pots in horizontal 
layers. Irrespective of the method of packing, each piece should be 












22 


No. 141—CASEHARDENING 


kept iy 2 inch from adjacent pieces and 2 inches from the pot walls. 
This clearance, which may seem excessive, allows for any settling of 
the mixture while in the furnace, for if the pieces should come in 
contact with each other, the penetration would be retarded and the 
surface would be defective at that point. A layer of mixture 2 inches 
deep should be placed in the bottom of the pot and be thoroughly 
tamped down; every successive layer should also be tamped. The pot 
cover should be thoroughly sealed or luted with fireclay. 

Large pieces which are too long for pots may be packed in pipe, the 
latter having a cap on each end, the threads of which have been 
coated with graphite to facilitate removal. With this arrangement, 
all moisture must be excluded from the mixture to prevent the forma¬ 
tion of steam which might result in an explosion. The pots should 
be spaced in the furnace so that 2 inches of space is available for the 
circulation of heat around every part of the pot surface. Even with 
this precaution, the pot nearest the furnace door will not be heated 
exactly the same as those farther back, and this should be allowed for. 
The best material for the pots is either cast steel or white iron. 

The Carbonizing- Heat 

There is a difference of opinion as to the proper temperature for 
carbonizing. Temperatures ranging from about 1600 degrees F. to 
1800 degrees F. are used quite generally. The writer has found that 
1725 degrees F. is a safe heat, which does not endanger a good steel. 
Higher temperatures may be used, but while the duration of the 
“run” is shortened, the quality of the work is likely to be impaired. 

For the accurate measurement of this temperature, only the very 
best make of pyrometer should be employed, those of the high-resist¬ 
ance type being preferable. They have the great advantage of sim¬ 
plicity, as the wiring does not need to be especially calibrated for 
each location, within reasonable limits, and it is possible to attach 
recorders to the same circuit without affecting the indicator reading. 
The pyrometer should penetrate the furnace wall and be placed as 
near to the work as possible, without liability of actual contact. When 
located vertically, the life of the porcelain jackets is increased, but 
the temperature of the cold junction is kept higher owing to the direct 
radiation from the top of the furnace. By inserting a pyrometer of 
sufficient length horizontally into the furnace wall, the cold junction 
may be kept at a lower and more uniform temperature. 

At the beginning of the heat the temperature may be kept, say, 50 
degrees F. higher than normal, until the heat begins to penetrate the 
work, when it should be reduced to normal. No fixed rule can be 
given for the time of complete saturation, as the size, shape and 
thickness of the pot wall, as well as the size of the work and kind 
of mixture, are determining factors. The duration of the run can 
be determined only by actual trial, depending upon such factors as the 
nature of the steel being treated, the carbonizing material, the degree 
of temperature and depth of case required. The best practice is to 
put a trial piece of the material into one of the pots in each furnace 


CASEHARDENING 


23 


and remove this piece about an hour before what experience has shown 
to be ample time. This trial piece is then heat-treated, as described 
later, and the depth of the hardened,case of a cross fracture is ob¬ 
served. A very accurate estimate can then be made as to the time 
to remove the rest of the work. 

With mixture “B” (see table given on preceding page) twelve hours 
will produce a case about 5/64 inch deep on a 1-inch shaft of 0.15 
per cent carbon content, with a temperature of 1775 degrees F. for the 
first two hours and 1725 degrees F. for the remainder of the run. A 
complete temperature record of each run in each furnace should be 
kept, the temperature being tabulated at least every twenty minutes. 
A convenient blank for this tabulation is shown in the accompanying 
illustration. The temperatures are first recorded in the outer radial 
spaces, and if the run exceeds twelve hours’ duration the inner spaces 
can then be used. Uniformity of temperature is, of course, a very 



Blank for recording Furnace Temperatures 


important factor and no matter what type of furnace or fuel is used, 
more or less regulation is necessary. A brief description of the 
method employed at the Hyatt Roller Bearing Go.’s plant to effect 
this control is of interest. 

4 

Colored Light System of Controlling Furnace Temperatures 

In the Hyatt plant, there are approximately twentynfive large car¬ 
bonizing furnaces fired with fuel oil and over one hundred semi¬ 
automatic gas furnaces for heating alloy steels. The gas furnaces 
are supplied by a battery of gas producers having a total capacity of 
2500 horsepower. The physical laboratory is located on one of the 
floors of the heat-treating building, and the temperature control room 
is a part of this laboratory. The furnaces are distributed on the 
four floors all over the building. Each furnace has a pyrometer which 
is connected by wires in an iron conduit, with the temperature control 
office. Over each furnace there is a small signal board with three 










24 


No. 141—CASEHARDENING 


lights; one white, one red, and one green. These lights are also 
connected with the temperature control office. All these circuits lead 
to a marble switchboard which has three signal lamps for each fur¬ 
nace, these being in series with corresponding lamps over that par¬ 
ticular furnace. There are two operators at the switchboard, each 
one having control of the furnaces of two floors. 

By touching a key, the operator can set the pyrometer of any fur¬ 
nace. If the temperature indicated by the pyrometer is normal, or 
within the allowable tolerance, an observation of the next one is 
taken, and so on. If the temperature is found to be too high, by 
touching another key the light over the furnace is changed to red, 
and if it should be found to be too low, the light is changed to green. 
At the same time, the corresponding light on the furnace signal board 
changes. Each attendant has under control six furnaces, and by 
simply looking at the signal boards he can determine readily whether 
the temperatures are high, low, or normal, and make the necessary 
regulations. After signaling a red or green light, the switchboard 
operator soon returns and takes another reading of the furnace to 
ascertain if the proper regulation has been made. 

Heat-treating- after Carbonizing 

When the steel emerges from the carbonizing furnace, it is of a dual 
nature, the case containing, say, 0.80 to 1.25 per cent carbon and the 
core 0.10 to 0.20 per cent carbon. Consequently, the carbonizing tem¬ 
perature is considerably above the critical range of both the case and 
the core, especially the latter, and as the duration of the run is several 
hours large crystals form readily in both case and core under these 
conditions. To restore the steel to the best grain size, two heats are 
required; a high one for refining the core, and a lower one for refining 
the case. 

Of course, it is possible, as well as quite common, to quench the 
work directly from the pot, using this heat as the first or “core heat” 
and then following with a second or “case heat,” or the second may 
be omitted entirely. In the latter instance, the temperature is too high 
to produce the best refinement of the case, although the core structure 
may be satisfactory. Again, if the work is allowed to cool so that the 
temperature is about right for the case, the core will not be perfect. 
Another disadvantage is due to delays in getting the work from the 
pot to the quenching medium, the pieces cooling more or less, and 
•thus giving results that are far from uniform. In view of these facts, 
it is greatly advantageous to apply two heats after the work has 
•cooled from the carbonizing temperature. 

We now come to the important question as to what are the correct 
heats. There is apparently a great difference of opinion on this 
subject, owing to such varying factors as the rate of penetration of 
different quenching mediums, the effect of the mass of the article 
quenched, and the temperature of the quenching medium. As previ¬ 
ously mentioned, the first heat is usually between 1600 and 1800 de¬ 
grees F. The second heat varies between 1375 and 1475 degrees F. 


CASEHARDENING 


25 


If a single heat is used, the range will vary still more, running high 
or low, according to whether the quality of the case or core is to he 
sacrificed. 

Quenching- 

Perhaps nothing connected with the heat-treatment of steel is of 
more importance than quenching. Slight variations in the angle of 
immersion often result in distortion of the work, apparently out of 
all proportion to what might be expected. Shafts should be im¬ 
mersed vertically and moved in the same position up and down until 
the quenching is complete. No matter how large the work, the re¬ 
sults obtained will pay for any special apparatus necessary to carry 
out this method. 

Where the maximum hardness is desired and only one heat is to 
be taken, water will give the best results, but the danger of dis¬ 
tortion is greater than when oil is used; this applies even more to 
brine and cold water. When two heats are employed, it is advisable 
to quench first in oil, for as the first temperature is higher, distortion 
is more likely to occur. After the second heat, the work may be 
quenched in water. Good results may be obtained by using water in 
both instances, as far as the structure of the steel is concerned, but 
there would be greater danger of distortion. When oil is used, care 
should be taken to maintain its temperature fairly constant. 

The advantage of drawing after carbonizing is much disputed. 
Opponents of this method claim that the core should have the 
requisite toughness and that any temperature which would toughen 
the case would soften the core. While this may be true to a certain 
extent, it has been found that in some instances good results can be 
obtained by drawing at about 400 to 450 degrees F., as this tem¬ 
perature relieves the strain in the case somewhat, without materially 
sacrificing its hardness. 


CHAPTER III 


CASEHAUDENTNG- ROLLER BEARING PARTS 

The manufacture of bearings of either the ball or roller type has 
grown remarkably in the past few years. The automobile industry 
is responsible for a large part of this growth, as it would havebeen 
difficult, if not impossible, to make the automobile a success without 
these anti-friction bearings. They must be made as small and light 
as possible and, at the same time, strong enough to carry the load 
of the car. 

The first essential of either type of bearing is a steel that possesses 
the qualities that will enable it to withstand the strains to which 
the bearings may be subjected. The second essential is correct ma¬ 
chining operations, as the parts must be made within a fraction of a 
thousandth of an inch of the correct size. The third essential —and 
perhaps the most important one — is the heat-treatment that these 
parts receive. This must be of such a nature as to make the steel 
resist the crushing, vibrational, frictional, or other strains to which 
all bearings are submitted. 

Annealing' 

After the steel parts of bearings have been machined to their cor¬ 
rect size and shape, it is essential that they be thoroughly annealed 
before they are carbonized, hardened and tempered. This relieves any 
internal strains, making the physical properties the same in all parts 
of a piece. Sometimes steel is annealed before any machine work 
is performed. This first annealing is usually done at the steel mill 
before the steel is shipped, but it will not take the place of the an¬ 
nealing that should be done after the steel is manufactured into the 
different pieces. 

In heating steel, the temperature will reach a point at which there 
occurs a change in the grain, or a new grain structure is born. With 
this rearrangement of the grain structure any unequal strains will 
disappear. Annealing consists of raising the temperature of the steel 
high enough to be assured that the metal has had time to thoroughly 
complete this change of structure. The steel is then allowed to gradu¬ 
ally cool to the temperature of the atmosphere. The slower this 
cooling takes place the more thorough will be the annealing and the 
obliteration of any internal strains. If cooled too quickly, a harden¬ 
ing of the metal takes place and other strains are likely to be set 
up; this is especially true if it cools unevenly or more quickly in 
some sections than in others. If the steel is heated too high above 
the transformation point the grain will become coarse, as each degree 
of temperature above this point adds to the coarseness of the grain 
structure. 


CASEHARDEXIXG 


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28 


No. 141—CASEHARDENING 


This information is the basis of three rules that it is well to follow 
when annealing all steels. First, heat the metal to a temperature 
above the highest transformation point, but not far enough above to 
coarsen the grain; second, hold the temperature at this point long 
enough to allow the transformation to be completed but do not pro¬ 
long it beyond that; third, make sure that the rate of cooling is suf¬ 
ficiently slow to prevent even a superficial hardening. 

Carbonizing - 

Some parts of bearings withstand the strain much better if made 
from low carbon steel and carbonized than if made from high carbon 
steel. This gives the outer surfaces of such parts a high carbon 
content that can be made hard to resist frictional wear or any tend¬ 
ency to crush or deform, while the low carbon center, or core, will 
remain soft and ductile, and thus make it difficult to break the piece 
with any of the severe shocks or strains it receives when in use. 
This carbonizing is done after the piece has been machined to the 
proper size and shape. Grinding is the only kind of work that is 
practical after the steel has been carbonized. 

In performing the carbonizing operation, the work is packed in 
iron pots and heated in furnaces in the same manner as already 
described in preceding chapters. Fig. 1 shows the bank of furnaces 
used by the Timken Roller Bearing Co., Canton, Ohio, for carbonizing. 
The first of these is a Frankfort furnace, which was installed by the 
Strong, Carlisle & Hammond Co., while the others were built by the 
Brown & Sharpe Mfg. Co. The special annealing furnaces built by 
the company for their own use are shown in Fig. 2. 

In the lower left-hand corner of Fig. 2 will be seen part of a pot 
that has been sealed up ready to insert in the furnace with the two¬ 
wheeled truck. Beside it are two pots that are only partly filled, 
indicating the way the work is laid in the carbonizing material. Each 
steel piece is kept at least one inch away from all other pieces. Then 
the carbon can penetrate all parts of the outer surface when the pot 
and its contents are heated to a temperature that is high enough to 
make the steel so hot that it will absorb carbon. 

The amount of carbon that is thus injected into the outer surfaces 
of the steel and the depth to which it penetrates are governed by 
the composition of the steel; the nature of the carbonizing material; 
the temperature of the furnace; and the time the work is held at this 
temperature. The outer shell of the steel piece is usually made to 
absorb enough to give it 1.00 per cent of carbon. This percentage of 
carbon gradually diminishes toward the center of the piece until the 
low carbon content of the original metal is reached. This will vary 
from 0.10 to 0.20 per cent of carbon, as it is such grades of steel that 
are used for parts that are to be carbonized. Many of the high- 
grade alloy steels, as well as the ordinary carbon steels, are car¬ 
bonized to various depths and with various percentages of carbon. 
These alloy steels give much better results in bearings than any of 
the carbon steels, and are nearly always used for the parts of bear- 


CASEHARDENING 


29 



Fig. 2. Furnace with Floor-plate removed to show Burner 























30 


No. 141—CASEHARDENING 


ings that have to withstand great strains. They are more expensive, 
however, and hence some of the cheaper bearings are made from 
the carbon steels. 

The higher the temperature to which the steel is raised, the more 
quickly will it absorb carbon to a given depth; the larger will be the 
amount of carbon that it will absorb in a given time; and the 
greater the depth to which it is possible to make the carbon penetrate. 
Owing to the grain of the steel becoming gradually coarsened by each 
degree that the temperature is raised above the transformation point, 
there is a limit to the heat that can be successfully used when car¬ 
bonizing steels. When heated too far above the transformation point 

the grain begins to crystallize, and at still higher heats the crystals 
begin to separate from one another. When these small cracks appear 
between the crystals the usefulness of the metal has been destroyed 
and it can only be restored by remelting and rerolling or reforging. 
Before the crystals begin to separate, the original fineness of grain 

can be restored by allowing the steel to cool and afterward heating 

it to a little above the transformation point and then suddenly cooling 
it by quenching in oil or water. This is the hardening operation 
and if properly done it will put the steel in as good a condition as 
though the grain had not been coarsened by overheating. 

Therefore, carbon steels with a transformation point of 1500 degrees 
F. or below can be held at a temperature of 1650 degrees for several 
hours, when carbonizing them, and the coarsened grain can then be i 
brought back to its greatest degree of fineness by the subsequent 
hardening operation. Some of the alloy steels with a transformation 
point of 1650 degrees F. can be heated to 1750 tdegrees .for the car¬ 
bonizing operation. These temperatures will give a rapid penetration 
of carbon; they also give as high a percentage in the outer shell as 
is required for practical work and to the necessary depth. Under 
ordinary conditions, a temperature of 1650 degrees F. maintained for 
one hour will cause the carbon to penetrate to a depth of 1/64 inch; 
three hours will give a depth of case of 1/32 inch; nine hours a depth 
of 1/16 inch; and twenty-four hours will give a depth of 1/8 inch. 
It is very seldom that the high carbon content is required for a depth 
of more than 1/16 inch and thus each lot of steel parts can usually 
be carbonized in a day’s run. 

After deciding what is the best temperature for the carbonizing 
heat, the best results can only be obtained by keeping the furnace 
uniformly at that temperature during all the time the work is in 
the furnace. Hence the same pyrometer is installed on these fur¬ 
naces that is used on the annealing furnaces and on all other furnaces 
used in the heat-treating department. Then the time will decide the 
depth of penetration, and when the desired depth has been obtained 
the pot should be taken out of the furnace and allowed to cool slowly 
to below 600 degrees F. After that, the work can be taken out of 
the pot and allowed to cool quickly. Then it is ready for any hard¬ 
ening and tempering that is required. In Fig. 3 is shown the way 
the work is removed from the pot and sorted. The square pots are 


CASEHARDENING 


31 


used for carbonizing work and the round ones for annealing. Thus 
there is no excuse for getting the work mixed in these two operations. 
Some take the pot out of the furnace and dump the work directly 
into the quenching bath for the hardening operation. The carbonizing 
temperature is usually too far above the transformation point, how¬ 
ever, to give the steel the finest grain that it is capable of assuming. 
Thus it is far better to allow the work to cool down from the car¬ 
bonizing temperature and then reheat it for the hardening operation. 

Hardening- 

In hardening carbonized bearing parts, the best results are obtained 
by giving them a double heating and quenching to get the proper 
degree of hardness in both the hard outer shell and the soft core. 



Fig. 3. Dumping and sorting Work that has been carbonized 


This is due to the fact that the transformation points of high and 
low carbon steels occur at different temperatures, which are often 
200 degrees apart. The steel should first be quenched from the high 
transformation point of the low carbon steel in the core and then 
reheated to the lower transformation point of the high carbon steel 
of the outer shell and again quenched. This produces bearings that 
it is almost impossible to break or crush under the load that they 
are designed to carry. It also gives them a wearing surface that 
will last a long time, as its grain is very fine and dense. 

Some parts of the bearings are made from steel that contains the 
desired amount of carbon. These parts are hardened and tempered 
without being subjected to the carbonizing process. Such work is 
inserted in furnaces and heated to the hardening temperature without 
being packed in iron pots. When it has reached the correct heat 
for hardening, it is quenched in tanks of oil or water. If the harden- 


















32 


No. 141—CASEHARDENING 



Arrangement of Hardening Furnaces and Quenching Tanks 






















CASEHARDENINCx 


33 


ing temperature has been just right, i. e., just above the transforma¬ 
tion point, the steel will then be in the hardest state to which it can 
be brought. It will be what is termed “glass hard,” and hence brittle, 
and must be heated again to a high enough temperature to draw out 
enough hardness to obtain the desired degree of toughness. 

The hardening temperature is the most important one and will 
not allow of as much variation as the annealing and carbonizing tem¬ 
peratures, if the best results ai;e to be secured. If the steel is not 
heated up to the transformation point before it is quenched, the 
change in grain structure will not take place and the steel will be 
no harder than when it leaves the rolling or forging operations. If 
heated above the transformation point the grain coarsens in propor¬ 
tion to the number of degrees of temperature above this point. As it 
coarsens, the physical properties of the steel deteriorate. Each rise 
of 50 degrees F. above the transformation point will lower the elastic 
limit of carbon steels something like 5000 pounds per square inch, 
and other physical properties in like proportion. Thus the greatest 
strength that can be given the steel can only be obtained at a certain 
temperature. Any variation from this means weaker metal. Its 
capacity to resist fatigue also loses somewhat over 15 per cent with 
each 50 degrees'rise above the transformation point. 

In hardening the various parts of Timken roller bearings, furnaces 
similar to those shown in Fig. 4 are used. These are the Strong, 
Carlisle & Hammond Co.’s furnaces. When the correct temperature 
has been obtained, the work is thrown into one of the tanks shown at 
the right, so it will suddenly cool. The hot steel would naturally 
raise the temperature of this bath, and when raised too high the steel 
would not be cooled quickly enough to produce the greatest degree of 
hardness. To overcome this difficulty, the liquid is kept constantly 
in circulation by allowing the hot fluid to overflow at the top of the 
tank and run into a larger tank which is located below the level of 
the floor at a considerable distance away. This allows the liquid to 
cool by radiation, and it is then pumped back into the tank. The 
stream than can be seen steadily flowing into the tank is the cool 
liquid coming from this pump. A wire basket lies in the bottom of 
the tank and covers nearly the entire bottom. When the hot steel 
is thrown in. the bath it is caught in this basket. The block and 
tackle shown is used to raise the basket out of the tank when it 
becomes filled with work which has had time to cool. The basket is 
merely raised above the level of the tank and the work dumped into 
cars or trucks that convey it to the other parts of the shop where 
it is to be used. Thus large quantities of work can be handled during 
a regular work day at a very small cost. 

The accuracy of the drawing temperatures is also very important. 
When too much of the hardness is drawn out, the bearing parts will 
be too soft and will compress when under load. When too little of 
the hardness is drawn out, they will be too hard and brittle and thus 
likely to crush or break. To get the greatest accuracy in the drawing 
temperatures, the furnaces shown in Fig. 5 are used. The heating 


34 


No. 141—CASEHARDENING 







K wf' : ' xf 

* pi 

m- 




sw _ 



Fig. 5. Rotary Furnaces used for drawing the Temper of Roller Bearing Parts 


















CASEHARDENING 


35 


chambers of these furnaces are revolving retorts through which the 
work slowly travels until it has absorbed the highest temperature of 
the furnace. It then drops out at the far end into receptacles which 
are removed when they are full. The retort has a spiral web on the 
inside and this, in connection with the speed at which the retort is 
revolving, controls the time that is consumed for the work to travel 
through. The work is dumped into the round hole shown at the near 
end of the furnace. Thus the tempering is done automatically and 
the furnace operator can devote most of his time to adjusting the 
burners, so that the furnace will be maintained at the correct draw¬ 
ing temperature. These furnaces can also be used for automatically 
heating bearing parts to the hardening temperature and then dropping 
them into a quenching bath at the far end of the furnaces. The retort 
would then have to travel slower to allow the work more time in 
which to reach this higher temperature. 


CHAPTER IV 


NEW CASEHARDENING- METHODS 

During the past decade there has been a great deal of investigation 
relating to the conditions under which the various carbonaceous gases 
may be used in place of the familiar solid carbonizing materials. The 
old well-known casehardening process described in the preceding chap¬ 
ters was the only one known for many centuries. It was used Without 
a question of its superiority until the manufacturer of armor plate 
became such a large industry that efforts were made to find a better, 
or cheaper, way of causing the carbon to penetrate this plate. 

The first method employed was to place an armor plate in a pit 
and cover it with a layer of charcoal, and then lower another plate 
onto it. The cover was then put on the pit and the plates heated to 
(or baked in) a temperature that was sufficient to cause them to 
absorb the carbon from the charcoal. Gas was generally used for 
the fuel, owing to the ease of controlling the heat. The next method 
tried was to send a current of carbonaceous gas between the two 
plates, in place of the charcoal. This caused the carbon to “soak in” 
in less time and was found more economical. Later, electricity was 
used for heating the plates, and with the carbonaceous gas and elec¬ 
tricity, the carbon penetration was found to be more uniform over 
the entire surface of the plate. 

The results obtained from the action of carbonaceous gas on armor 
plate have been such that a muffle carbonizing furnace has been built 
and placed upon the market. This machine, illustrated and described 
in detail in succeeding pages, holds the work in a revolving retort, 
through which is sent a current of carbonaceous gas. This retort 
serves as a muffle that is surrounded with the flames of the heating 
gases. With this furnace, small pieces can be carbonized in much 
less time than formerly, and at about one-half the cost as compared 
with packing in iron boxes and then baking in an oven furnace. All 
of the labor of packing materials is done away with; carbon will 
penetrate the metal in less time and more evenly; its depth and 
percentage can be controlled more easily; and the work can be heated 
to the carbonizing temperature more quickly and maintained there 
more easily. A steady flow of carbonaceous gas can be kept passing 
through the retort, and thus any depth of carbon can be obtained 
without repacking the work. 

Comparison between Old and New Methods 

When considerable depth of carbon is required, this is impossible 
with the old method of packing with bone and charcoal in an iron 
box and sealing on the cover. This is due to the fact that only a 
certain amount of carbon is present, and the longer the work is baked. 


CASEHARDENING 


37 


the more there will be in the steel and the less in the charcoal. When 
an equilibrium is established, no more carbon will penetrate the 
metal, and to obtain a greater depth, the work must be. packed in 
fresh carbonaceous material and the heating repeated. 

With the gas process, however, the percentage of carbon in the 
gas surrounding the work can' be maintained at a permanent figure 
until the carbon has penetrated to the center of the metal, the per¬ 
centage of carbon possible to impart to the steel being far above that 
which is used for any kind of commercial work. Some of the gases 
that have been experimented with are methane, ethylene, illuminating 
gas, carbon monoxide, carbon dioxide, and gases that are made from 
liquids like petroleum, naphtha and gasoline. Most of these gases 
have been used in combination with ammonia, in order to ascertain 
to what extent this would aid in the penetration of the carbon. 

From the numerous experiments that have been conducted, it has 
been found that carbon monoxide is far superior to any of the solid 
carbonaceous materials in the specific, direct carbonizing effect it 
has upon steel. It is also better than all other gaseous materials in 
this respect. Carbonizing materials that do not contain nitrogen cost 
only from one-tenth to one-twentieth of the nitrogeneous materials. 
It has been found, however, that nitrogen acts as a carrier for the 
carbon, and when it is not present, carbonaceous materials have a 
very weak carbonizing, effect; some investigations have shown that 
the effect is absolutely nil without the intervention of gaseous carbon 
compounds. When solid carbonaceous materials are used, the specific 
effect of the nitrogen is very weak, and it is only when these contain 
a high percentage of the cyanogen compounds that they have any 
marked carbonizing effect. 

While carbon monoxide is capable of rapid penetration, it has an 
oxidizing effect on steel, and is liable to form a scale that will spoil 
small work which cannot afterwards be ground. This oxidizing ef¬ 
fect is more pronounced in chromium and manganese steels. When 
carbon monoxide alone is used for the carbonizing medium, there is 
a distinct demarkation between the carbonized zone and the core of 
the metal. This is also a detrimental feature, in that when the piece 
is hardened, it has a tendency to crack at this demarkation, causing 
the outer shell to peel off. 


The Giolitti Process 

To overcome these bad effects of carbon monoxide, a new process 
has been developed by Dr. F. Giolitti, Genoa, Italy. In this process 
the work is packed with wood charcoal in a cylinder, and when heated 
to the carbonizing temperature, a current of carbon dioxide is in¬ 
jected into the cylinder. It was demonstrated that when a slow 
current of carbon dioxide traversed a mass of wood charcoal, the 
carbonizing gas was supplied with great rapidity and without any 
excess of carbon monoxide. Thus, an equilibrium with free carbon 
was established at the carbonizing temperature. The exhaust gas 
contained less than three per cent of carbon dioxide, it being almost 


38 


No. 141—CASEHARDENING 


entirely carbon monoxide, and its volume being about double that of 
the carbon dioxide which was introduced into the apparatus. Some 
results that were obtained with carbon monoxide alone, and in 
combination with charcoal, are shown in Table I. 

With the use of this new process, a more rapid penetration can be 
obtained than with any of the solid or gaseous materials, except pure 
carbon monoxide. The carbon is evenly distributed in the carbonized 
zone, and the peeling of the outer shell, when hardened or tempered, 
is reduced to a minimum. Any desired depth of penetration can be 
obtained without renewing the carbonizing material, and there is 
absolute security against the introduction of any foreign substance. 
Variations in the percentage and depth of the carbon can be obtained 
by diluting the carbon monoxide in nitrogen; by limiting the contact 
of the solids with the metal; and by varying the temperature during 
the carbonizing operations. Expansion or contraction and warping 
of the pieces being carbonized has also been reduced to a minimum. 
In fact, there are so many features that make it superior to the old 
method of packing and sealing the work to be carbonized in iron 
boxes, that it is safe to say that the new process is incomparably 
better. 


TABLE I. RESULTS OF CARBONIZING STEEL WITH CARBON MONOXIDE FOR 
TEN HOURS AT 2000 DEGREES FAHRENHEIT 


Depth from 
Surface at which 
Sample was 
Analyzed, Inches 


Percentage 

of Carbon 


Carbon Steel 

Nickel-chromium Steel 

CO Gas Alone 

CO a and Charcoal 

CO Gas Alone 

CO a and Charcoal 

1/64 

0.70 

1.17 

0.86 

1.16 

3/32 

0.67 

0.81 

0.87 

0.81 

3/16 

0.53 

0.34 

0.75 

0.50 

5/16 

0.39 

.... 

0.59 

.... 


After much experimenting, Dr. Giolitti decided that the double 
muffle furnace, shown in Fig. 1, was the most efficient and economical 
for carbonizing small or medium size pieces of varying shapes. By 
providing two muffles, one could be filled with work and kept at the 
carbonizing temperature, while the other was being emptied and 
refilled. If the amount of work would warrant, many more muffles 
could be used in the same furnace. The muffle to the right is shown 
by a sectional view through the center of the furnace, but the retort 
that holds the work is not sectioned, while the muffle to the left is 
shown by a sectional view on the center line, thus revealing the 
interior. 

Cylindrical muffles A are made of some refractory material and are 
built into the brick-work of the furnace. Surrounding them are 
passages B, in which the combustion of the heating gases take place. 
These passages are lined with firebrick or fireclay and the furnace is 















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~7~i ^ 


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. 




mm 
V -mm 


ymizm 




" If ' 

— 

r 


' 

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m 




Tig. 1. Muffle Tttm*c* for CMrbtmizfng St*#l with CWoo* 1 *.n<4 C ’itrhwit.'.wu* 0*4 










































































40 


No. 141—CASEHARDENING 


provided with, regenerators, so that the fuel gas, which is furnished 
by producers, can be used in the most economical manner. By this 
arrangement and the valves that are supplied, it is possible to main¬ 
tain the work at a uniform predetermined temperature. The work¬ 
holding retort C is made of seamless steel tubing and sets into flange 
D , which latter is attached to a frame that fits into the brickwork 
underneath the heating chamber. Flange E, which is made U-shaped 
to hold cover R, supports retort C at the top of the furnace. Thus, 
it is only the work of a few minutes to take out retort C and replace 
it with a new one, when it has warped out of shape or burnt through. 

Inside of retort C is located a hollow cast-steel cylinder F, .and in¬ 
side of this is located a device that evenly distributes the carbonizing 
gas around the work in the retort, by sending it through cover plate G, 
which is filled with holes. When all of the carbon is taken from the 
gas, it is allowed to escape through vent H. The work to be car¬ 
bonized is stacked up on cover plate G, which rests on casting F, and 
this, in turn, rests on the same flange D that supports the retort. To 
the bottom of this flange is attached the cast-iron nozzle I, which is 
closed at the bottom by the non-return valve J. Underneath the 
muffles are located two hydraulic rams K and a cylindrical iron tank 
L, mounted on wheels, for handling the solid carbonizing material, 
which is in a granular condition. Tank L may be turned on its wheels 
so that spout Q will come under the nozzle I of either muffle. 

In operating this furnace, a continuous method can be employed. 
When a batch of work has been carbonized, the granular carbon is 
drawn off through nozzle 1 into tank L. After that, pipe M, through 
which the carbonizing gas enters the retort, is unscrewed, and ram 
K raises steel pot F towards the top of retort G, thus pushing the 
work that rests on plate G up with it, so that it can be removed as 
the ram proceeds upwards. When casting F has reached the top of 
its stroke, which is a little below the top of the furnace, and the old 
work has been taken out, new work is placed on disk G, which is 
lowered by ram K as fast as the work is located. When the retort is 
filled with work, tank L is wheeled out from under the muffles and 
raised over the top of the furnace with a hoist on a swinging arm. 
When over the top of the furnace, pipe N at the bottom of tank L is 
lowered into opening 0 in the center of cover R. Valve P is then 
opened to allow the hot granular carbon to flow out of tank L and 
fill the interstices surrounding the work. 

If granular carbon is held at or near the carbonizing temperature, 
it acts very much like a liquid, and readily flows into all of the 
crevices surrounding the articles in the retort that are to be case- 
hardened. When it is drawn off at the bottom of the retort it is at 
the carbonizing temperature, and the time consumed in removing the 
finished work and replacing it with new is so short that the granular 
carbon does not cool down to a temperature below 1500 degrees F. 
Thus, it retains its mobility and flows around the work. An operator 
on top of the furnace might assist this flow by using iron rods that 
can be inserted into the retort through holes in the cover. 


CASEHARDENING 


41 


When the retort is 
properly filled, butterfly 
valve P is closed and 
tank L is lowered and 
wheeled to its position 
underneath the muffler. 
The work is then allowed 
to stand until the carbon¬ 
izing temperature has 
been reached. In the 
meantime, pipe M has 
been screwed into posi¬ 
tion, and when the car¬ 
bonizing temperature has 
been reached, the car¬ 
bonaceous gas is injected 
into the retort through 
this pipe. While the 
work in the retort to the 
left is being carbonized, 
the retort to the right 
can be emptied and filled, 
without in any way dis¬ 
turbing the process in the 
other. 

Another method that 
has been tested by the de¬ 
signers of this furnace is 
that of compressing the 
carbonizing gases, and 
some very good results 
have been obtained. The 
tests demonstrated that 
when carbon monoxide 
acts on ordinary steel in 
the presence of free car¬ 
bon, as in the furnace 
shown in Fig. 1, an in¬ 
crease in the depth of 
carbonization will be ob¬ 
tained with an increase 
of the pressure on the 
gas, and there will also 
be an increased concen¬ 
tration of carbon within 

Figr. 2. Carbonizing with Compressed Carbonaceous the carbonized zone> « 
Gas, using Electric Heating Means 


































































42 


No. 141—CASEHARDENING 


In carrying out some experiments of this nature, a cylindrical re¬ 
tort was used, like that shown in the sectional view in Fig. 2. In 
this, electricity was used to heat the work to the carbonizing tempera¬ 
ture. The work was packed in charcoal in a retort into which a 
current of carbon dioxide was injected, in a very similar manner to 
the method used in the furnace in Fig. 1. In the illustration, A and 
B are the clamps for the terminals and these conduct the current to 
nickel-wire spiral D. This wire is wound around porcelain tube E, 
which can easily be inserted into, or taken out of, the apparatus. By 
taking off nut N, tube E is readily slipped into fireclay tube F, which 
is surrounded by steel tube G and insulated with asbestos. 

The carbon dioxide gas enters the retort through tube C and the 
used gas escapes through pipe H. Porcelain tube I contains a thermo¬ 
electric couple that is inserted into the retort at the opposite end from 


TABLE II. RESULTS OF CARBONIZING STEEL WITH COMPRESSED 
GAS FOR THREE HOURS 


Kind of Steel 

Carboniz¬ 
ing Tem¬ 
perature, 
Degrees F. 

Pounds 
Pressure of 
Carboniz¬ 
ing Gas 

Percentage of Carbon 
at a Depth of 

1/64 Inch 

1/32 Inch 


1600 

235 

0.71 

0.12 

Nickel steel *. 

1775 

235 

0.99 

0.29 


1650 

400 

0.73 

0.36 


1750 

235 

2.22 

1.03 

Chromium steel t. 

1925 

235 

3.10 

1.39 


1875 

400 

2.37 

1.40 


1525 

235 

0.45 

0.54 

Chrome-nickel steel t. 

1650 

235 

0.76 

0.49 


1525 

400 

0.54 

0.56 


* Composition in per cent: Nickel, 5.02; carbon, 0.118; silicon, 0.20; manganese, 1.53. 
t Composition in per cent: Chromium, 2.33; carbon, 0.41; silicon, 0.15; manganese, 1.02. 
t Composition in per cent: Nickel, 3.17; chromium, 1.5; carbon, 0.33; silicon, 0.06; 
manganese, 1.15. 

gas tube C. With it the temperature of the entire length of the case- 
hardening chamber can be measured. Blocks L are the experimental 
pieces to be carbonized, and are surrounded by granular carbon. 
Table II shows some results obtained with various kinds of alloy 
steels. While this is only a crude experimental apparatus, it would 
seem to suggest some ideas or principles that can very profitably be 
used for carbonizing steel parts on a commercial scale. 

Definite proof was obtained that variations in the pressure of the 
carbonizing gas were always accompanied by variations in the depth 
of carbonization, and also in the percentage of carbon in the car¬ 
bonized zone. It was found, however, that when the pressure was 
too high it would cause an oxide to form on the steel and this was 













CASEHARDENING 


43 


more pronounced with chromium and manganese steels than with 
others. It was also found that as the carbonizing temperature was 
raised, the pressure could be increased without causing this oxide to 
form. Thus, the higher the carbonizing temperature, the higher can 
be the pressure used on the carbonizing gas, with an absolute assur¬ 
ance that no oxidation will take place. 

A rod of soft steel, 2% inches in length and three-eighths inch 
in diameter, was casehardened for about three hours by heating three- 
fourths of an inch of its central portion to about 1800 degrees F. and 
allowing this temperature to decrease towards the two ends, so that 
at these the temperature was about 900 degrees F. Unmistakable car¬ 
bonizing took place in all portions that were above 1450 degrees F. 
The surface was absolutely unaltered in the hottest portion in the 
center, which was also the most intensely carbonized, while a distinct 
layer of oxide was seen in the cooler portions, this oxide thickening 
as the temperature lowered towards the ends. 

Such good results were obtained by compressing the carbonizing 
gas as it was injected into the bed of charcoal in which the work was 
packed in the retort, that this method promises to become a com¬ 
mercial success. While the mixed agent, carbon monoxide and char¬ 
coal, increased both the speed of penetration and the percentage of 
carbon in the carbonized zone over all previous methods or materials 
used for carbonizing steels, compressing the carbon monoxide has 
still further increased these factors. Like all methods and processes, 
however, it must be handled properly. The amount of compression, 
as well as the carbonizing temperature, varies with different kinds of 
steels. Therefore, these must be discovered and properly adjusted, if 
work is to be turned out that is free from oxide and scale, and that 
has the desired penetration uniformly distributed over all portions 
of the exposed surfaces. 

Whether work is carbonized with an ordinary flow of carbon 
dioxide into and through the charcoal, or by compression, the ad¬ 
vantages which vertical muffles, as shown in Fig. 1, have over hori¬ 
zontal muffles are in the greater speed of charging and removing the 
work, due to the greater, simplicity of the operations, the uniformity 
of the treatment of all pieces forming the charge, and the more uni¬ 
form distribution of the carbonizing gases, due to the spaces being 
reduced to a minimum. 

Time Required for Operation 

The time for the various operations with the furnace shown in Fig. 
1 is as follows: Charging the pieces to be carbonized, from 1 to 5 
minutes, according to their size and shape; completely filling the 
retort with granular carbon, from iy 2 to 4 minutes; lowering ram K, 
replacing pipe M, removing tank L, and closing down cover R, 1 
minute; drawing the granular carbon from retort C into tank L, 4 
minutes; raising ram K and removing the work from the retort, 2 
minutes. The time consumed in all the operations, where ordinary 
work is being handled would, therefore, be about ten minutes, but 


44 


No. 141—CASEHARDENING 


with specially shaped pieces and unfavorable conditions, this time 
might be extended to 30 minutes. 

By pre-heating the work to a carbonizing temperature before put¬ 
ting it into the retort, no time will be lost in fully heating it in this 
furnace. The temperature of the granular carbon can then be main¬ 
tained nearly up to the carbonizing temperature, as it will not be 
chilled by cold work. The process can thus be made strictly continu¬ 
ous. Under these conditions, a depth of carbon penetration of 1/32 
inch can be given the work in one hour, and of 1/16 inch in two hours. 
Thus it will be seen that from 1% to 2 y 2 hours is all that is required 
for the complete carbonizing operations in one retort. 

By using gas for fuel and gas for carbonizing, the work can be con¬ 
trolled within closer limits than with any other process, unless it 
should be an electric one, and the arrangements of this furnace are 
such that its capacity for producing work is greater than that of any 
furnace which has been designed with the same size of work holder. 
If there is not enough work to keep both muffles going on pre-heated 
work, one of the muffles can be used as a pre-heating furnace, while 
the other is doing the carbonizing. By alternately using the muffles 
for preheating and carbonizing, an amount of work will be turned 
out that will compare favorably with any other casehardening fur¬ 
nace. If desired, the current of carbonaceous gas can be used for a 
whole or any given part of the carbonizing time, and thus the results 
obtained can be made to cover a wide range. Where localized case- 
hardening is required, the granular carbon can be drawn off until only 
enough is left to insure the chemical equilibrium in the gas, and by 
thus isolating the carbon monoxide, it will intensify its specific action. 

American Gas Furnace Co.’s Apparatus for Casehardening- by Gas 

The casehardening process described on the preceding pages is 
intended for specially large work. The American Gas Furnace Co. 
has brought out a casehardening plant, using gas as the carbonizing 
material, which is suitable for small and medium work. Briefly 
stated, the process as performed by the apparatus brought out by this 
company consists in placing the work in a slowly revolving, properly 
heated, cylindrical retort into which the carbonizing gas is injected 
under pressure. From the gas, the work absorbs the volatile carbon. 
The absorption of carbon begins as soon as the work is sufficiently 
heated to attract it, and continues throughout the process, because 
the work is constantly and uniformly exposed to a carbon charged 
atmosphere under pressure, instead of to solid carbonaceous material 
which turns to ashes wherever it is in proximity to the heated parts. 
All the parts of the same piece of work and all the pieces contained 
in one charge in the retort are, therefore, continuously subjected to 
exactly the same condition as regards the presence of carbon, and 
the result is a uniformity and speed of operation not obtainable by 
other methods. 

The complete gas casehardening plant consists of a generator of 
carbonizing gas and revolving retorts used for the carbonizing process. 


CASEHARDENING 


45 



Fig-. 3. Revolving Cylindrical Retort brought out by the American Gas Furnace Company for Casehardening by Gas 
















































































































































































































































46 No. 141—CASEHARDENING 

The generators are generally made in sizes to supply two or more 
machines with carbonizing gas. The gas is produced from refined 
petroleum and the carbon vapor is so diluted by a neutral gas that the 
proportion of carbon that is supplied to the work is not greater than 
that which can be absorbed by the work without forming obstructive 
carbonaceous deposits. The carbonizing machine proper consists of a 
carbonizing retort and a cylindrical furnace body, in which the retort 
is enclosed and in which it rotates. Suitable arrangement is made 
for charging and discharging the work. The furnace for the exterior 
heating of the retort is fired with fuel gas requiring a positive air 


Tig - . 4. General View of Cylindrical Retort for Casehardening by 
Gas, shown in Fig. 3 

blast. Besides these carbonizing retorts, of course, ordinary furnaces 
for reheating the work for hardening are required. This reheating 
can also be done in the carbonizing retort, if desired. 

The line-engraving, Fig. 3, shows a sectional view of the carboniz¬ 
ing machine. A wrought-iron retort is shown at A which is slowly 
rotated on rollers B by worm-gear G, which, in turn, is driven by 
worm D, the shaft of which is rotated in any suitable manner, pre¬ 
ferably by a sprocket and chain. At E are shown air spaces in the 
retort formed by two pistons I between which the work is placed. At 
F is shown the heating space surrounding the retort into which the 





CASEHARDENING 


47 


fuel gas and air are injected under pressure from two rows of 
burners, indicated in the upper half of the casting at G. The cover 
H closes the retort. It is connected to the piston I by pipe J, which 
also provides a vent for the retort. Cover H and this pipe are with¬ 
drawn to charge and discharge the retort, and are replaced after the 
work is inserted, before beginning the carbonizing process. 

Steel to Use for Gas Casehardening- 

For casehardening by the gas method, it has been found that 
articles made from machine steel containing from 0.12 to 0.15 per cent 
carbon give the best results, although steel containing from 0.20 to 
0.22 per cent carbon may also be used to advantage. The length of 
time that the work is required to remain in the carbonizing retort 
depends upon the depth of carbonized surface required. A thin shell 
will be produced in one hour, while the thickness will constantly in¬ 
crease if the work is left in the retort up to nine or ten hours. The 
treatment after the work is carbonized is the same as that which 
should be given to ordinary casehardened work. As already stated, it 
is rarely the case that work is properly hardened, if quenched directly 
from the carbonizing retort, but, as a general thing, it should be al¬ 
lowed to cool slowly and then be reheated to harden the carbonized 
surface at the proper hardening heat. 

The heat of the retort while carbonizing the work must be varied 
for different classes of steels, and the proper degree can only be de¬ 
termined by trial. The higher the heat, the quicker the carbon will 
be absorbed from the carbonaceous gas, but the higher heat tends to 
make the core coarse. As a rule, about 1500 degrees F. will be found 
a suitable temperature, and this should not be exceeded unless tests 
have been made to determine that higher temperature may be used 
without detriment to the structure of the steel. 

The gas casehardening process can be carried out more rapidly and 
more uniformly than is possible with solid carbonaceous materials. 
Another advantage is that the volatile carbon will find its way into 
slots, holes and cavities which could not receive the carbon from the 
granulated bone or any other solid packing material, and, hence, the 
uniformity of the product is greater. In many cases, low-carbon steel 
treated by the gas casehardening process may, therefore, be sub¬ 
stituted for tool steel in machine construction. 



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MACHINERY’S 

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Machinery’s Handbook comprises nearly 1400 pages of carefully edited and 
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It is the one essential book in a library of mechanical literature, because it 
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engineering practice. Price $5.00. (£1). 

GENERAL CONTENTS 

Mathematical tables'—Principal methods and formulas in arithmetic and algebra— 
Logarithms and lsgarithmic tables—Areas and volumes—Solution of triangles and 
trigonometrical tables—Geometrical propositions and problems—Mechanics—Strength of 
materials—Riveting and riveted joints—Strength and properties of steel wire—Strength 
and properties of -wire rope—Formulas and tables for spring design—Torsional strength 
—-Shafting—Friction—Plain, roller and ball bearings—Keys and keyways—Clutches and 
couplings—Friction brakes—Cams, cam design and cam milling—Spur gearing—Bevel 
gearing—Spiral gearing—Herringbone gearing—Worm gearing—Epicyclic gearing—Belting 
and rope drives—Transmission chain and chain drives—Crane chain—Dimensions of small 
machine details—Speeds and feeds of machine tools—Shrinkage and force lit allowances— 
Measuring tools and gaging methods—Change gears for spiral milling—Milling machine 
indexing—Jigs and fixtures—Grinding and grinding wheels—Screw thread systems and 
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twist drills—Heat-treatment of steel—Hardening, casehardening, annealing—Testing the 
hardness of metals—Foundry and pattern shop information—The welding of metals— 
Autogenous welding—Thermit welding—Machine welding—Blacksmith shop information 
—Die casting—Extrusion process—Soldering and brazing—Etching and etching fluids— 
Coloring metals—Machinery foundations—Application of motors to machine tools—Dynamo 
and motor troubles—Weights and measures—Metric system—Conversion tables—Specific 
gravity—Weights of materials—Heat—Pneumatics—Water pressure and flow of water— 

Pipes and piping—Lutes and cements—Patents. 


Machinery, the leading journal in the machine-building field, the originator 
of the 25-cent Reference and Data Books. Published monthly. Subscription, 
$2.00 yearly. Foreign subscription, $3.00. 


THE INDUSTRIAL PRESS, Publishers of MACHINERY 
140=148 LAFAYETTE STREET NEW YORK CITY, U. S. A. 

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